The present invention relates to a glassy base material of optical fibers or, more particularly, to a base material of optical fibers having an accurately controlled ratio of the core diameter and thickness of the cladding layer and a method for the preparation thereof.
A typical method for the preparation of a glassy base material of optical fibers composed of the core and the cladding layer therearound, the refractive index of the former being larger than that of the latter, in the prior art is described, for example, in Japanese Patent Kokai 55-32716 according to which a porous sintered body in a rod-like form is first prepared by the deposition of fine glass particles formed by the flame hydrolysis of a gaseous starting material containing silicon to form the core portion and the cladding layer therearound and the porous body is vitrified by heating into a glassy base material which is enclosed by fusion in a quartz glass tube to give a base material of optical fibers.
One of the important requirements in a base material of optical fibers is an accurately controlled diameter of the core portion and the thickness of the cladding layer or the ratio thereof. In the above mentioned prior art method, which is usually called the rod-in-tube method, quartz glass tubes not always have a sufficient accuracy in the diameter and wall thickness so that it is a usual practice to select and use a quartz glass tube which satisfies the requirements for the desired dimensions of the final product or, alternatively, to control the diameter of the glassy base material by drawing to fit the available quartz glass tube. Such a method is of course very complicated and disadvantageous as an industrial method for the mass production of standard products. Moreover, most of the commercially available quartz glass tubes are manufactured by the fusion of natural quartz so that numberless tiny bubbles are unavoidably contained therein to cause a great decrease in the mechanical strength of the optical fibers produced therefrom.